Injection devices (hereafter called merely "devices") are used in medicine to inject liquids for instance into the human body. The present application relates to devices constructed so that patients are able to administer to themselves the injections at any time. Significant criteria are the compactness of the device which must be carried on one's person, the possibility to carry out several injections using one ampoule in the device, and accurately preselected injection doses, as well as reliability and simple handling of the device.
Such devices are known. European patent document B10 293 958 describes one employing ampoules with plungers. The plunger is advanced by a rod driven by an electric motor and the injection liquid in the ampoule is thereby expelled from the ampoule and through a needle. A sensor measures the dose-setting adjustment of the rod and hence of the plunger. The electrical signal magnitude measured is compared at a comparator with an electrical reference value previously set by the patient and can be displayed. When the dose set by the reference value has been reached, an electric check is performed each time to see whether the set dose is still present in the ampoule. If not, the electrical motor is locked up to preclude injection and this state is then displayed.
While this device can be carried on one's person, it is fairly bulky because of the electric motor and the gear sets. Also, the sensor provides an analog output and if there were a change in amplification, the desired rod adjustment conforming to the reference value might differ from the actually set value and the patient would inject the wrong dose. The patient lacks checking means. Moreover, the battery condition is not displayed, and it might be at such a Level, previous to a required injection, adequate still to control the device and feed the displays but insufficient to lower the motor and therefore the injection could not be carried out. The complexity of the electronic control of this device includes further functions not cited herein and therefore it is comparatively large.
A further device is known from WO 87/02895. It is actuated solely manually and has the bulk of a fountain pen with clip; in other words, it is easily carried on the patient's person. The injection dose can be set at a rod by rotating an operating head; this rod moreover does not touch the ampoule which is used in this device and which is filled with the injection liquid. When the operating head is displaced axially, the rod is made to touch the plunger and the adjusted dose is then expelled through a needle by said plunger. Thereupon the rod and operating head snap back and the next dose can then be set by rotating said operating head. The size of the dose is determined by the number of revolutions undergone by the operating head and illustratively may be detected by acoustic signals emitted every quarter turn.
The set dose can be checked acoustically only during setting, but not later. This constitutes to a substantial drawback because erroneous doses cannot be ruled out.